1. Field of Invention
The present invention relates generally to variable breadth centrifugal pumps (also referred to as variable capacity centrifugal pumps) and, more particularly, to providing an improved shrouded impeller constructed and arranged to provide a specific pressure head at pump shutoff operating condition.
2. Brief Description of Related Art
Standard (fixed geometry) centrifugal pumps are designed to operate at peak efficiency at a specific pressure head and flow rate. In a standard centrifugal pump, such as for example, a Navy Standard Fire Pump, the volume of water within the pump flow passages, i.e., the fluid passages between adjacent impeller vanes, is fixed by the area defined by adjacent impeller vanes, the impeller wall, and the opposing wall of the pump casing. At a constant drive shaft speed (constant impeller speed), as the demand on a standard centrifugal pump decreases the flow rate decreases causing a corresponding increase in the pressure head. In certain pump applications, drive shaft speed, and thus impeller speed, may be varied to account for reduced loads. However, pump efficiency decreases with decreasing impeller speed.
Firemain systems aboard surface ships often include multiple centrifugal pumps connected in parallel, with one or more of the pumps run continually to maintain a specific system pressure and to ensure quick response in the event of an emergency. A typical Navy ship firemain system includes six Navy Standard Fire Pumps, standard centrifugal pumps each capable of producing a constant flow rate of 1000 gallons per minute at a pressure of 150 pounds per square inch. The continuous demand on the firemain system, however, is generally for a flow rate much lower than the design flow rate of the individual pumps. Firemain loads can be less than 25 percent of the design flow rate of the individual pumps, resulting in firemain pressures that are much greater than the design pressures of the pumps.
Increasing firemain system pressure reduces pump efficiency and reliability. Increased pumping pressure results in higher water velocities which, in turn, may produce leaks, increase system noise, and cause corrosion and erosion damage to connected equipment. On naval vehicles powered by gas turbine engines, varying pump shaft speed is not an option. Consequently, centrifugal pumps used on such naval vehicles are run at a constant impeller speed. When pump demand drops below the pump's design flow rate, the result is pressures that are much greater than the design pressures of the pumps. Thus, a need exists for centrifugal pumps which are capable of varying flow rates at a constant pressure and constant impeller speed. Such variable capacity pumps may, therefore, be employed in multiple centrifugal pump systems to efficiently and quietly vary flow and pressure characteristics to match varying system demands.
Variable capacity centrifugal pumps (VCCP) have been designed by the U.S. Navy for use as the firemain system's lead pump maintaining constant system pressure. Variable capacity centrifugal pumps are described, for example, by U.S. Pat. Nos. 4,828,454 and 4,417,849, both assigned to the U.S. Navy. A typical approach for providing a centrifugal pump with variable capacity is to vary the width of the impeller flow passages by incorporating axially adjustable impeller sections.
In U.S. Pat. No. 4,417,849, the variable capacity centrifugal pump arrangement includes two intermeshing impeller sections mounted to a common pump shaft such that one of the impeller sections is axially movable relative to the other impeller section. By axially adjusting the relative position of the impeller sections, the width of the impeller flow passage is varied to increase or decrease flow rate in response to system requirements. U.S. Pat. No. 4,828,454 describes a variable capacity centrifugal pump wherein the flow rate (capacity) is controlled by a shroud movably attached to the impeller drive shaft. The axially movable shroud has grooves for receiving individual impeller vanes. By axially adjusting the the relative position of the impeller and shroud, the width of the impeller flow passage is varied to increase or decrease flow rate in response to system requirements.
During the low demand periods typical for Navy firemain systems, only the variable capacity centrifugal pump (VCCP) is needed to satisfy demand. The VCCP maintains a system pressure of 150 pounds per square inch over a flow range of approximately 250 to 1000 gallons per minute. During intermittent operations requiring additional capacity, such as during deck wash down, pumping of bilges, or in emergency situations, one or more of the stock centrifugal pumps is brought on line to satisfy the increased demand. During periods of increased demand, the stock pumps operate at their design point (1000 gallons per minute at a head of 150 pounds per square inch for Navy Standard Fire Pumps) with the VCCP adjusting its flow rate to provide the balance of the flow demanded while maintaining a head of 150 pounds per square inch. When the increased demand subsides (e.g., dick wash down hoses are turned off), the flow rate will decrease and, ideally, the stock pump(s) will be taken off line and the VCCP will adjust its flow rate to satisfy the reduced demand.
However, it is often the case that, as demand subsides, the stock pump continues to operate resulting in increased system pressure. As long as the VCCP can adjust its flow characteristics to match the increased system pressure, the VCCP and the stock pump share the load. As required flow rate decreases, pump head increases, until a maximum head is reached at shutoff, i.e., zero flow for a standard centrifugal pump. If the VCCP has a lower value of shutoff head than the stock pump then at some point along the head-capacity curve the head of the VCCP will fall below the head of the stock pump. At this point, the stock pump will begin to provide all the flow demanded by the system. As the VCCP continues to operate, undue heating of the fluid within the VCCP will occur ultimately resulting in failure of the VCCP. Thus, in multiple pump systems, there exists a need for a VCCP capable of being adapted to a specific shutoff head that matches the shutoff head of the stock pumps in the system.
The pumps disclosed in U.S. Pat. Nos. 4,828,454 and 4,417,849 were intended to meet operational requirements for Navy variable capacity centrifugal pumps. The pumps were designed to provide constant discharge pressure over a wide operating range, typically on the order of 250 to 1000 gallons per minute. However, an additional requirement for Navy variable capacity centrifugal pumps includes a constantly rising head-capacity curve such that the shutoff head of the variable capacity centrifugal pump matches the shutoff head of a Navy Stock Fire Pump. Present variable capacity centrifugal pump designs do not provide the capability of adapting to and matching the shutoff head of a specific stock pump and, therefore, do not meet the full operational requirements for Navy